The present invention relates to vacuum filters of the type wherein a filter tank is divided horizontally into an upper dirty liquid tank and a lower filtrate compartment beneath a filter medium, e.g., paper, which is interposed between the two compartments. To renew the filter paper, a chain and slat conveyor is utilized to move the paper and the contaminants accreted thereon out of the tank by travel up an inclined end wall of the tank. The conveyor is actuated to renew the filter medium in response to the differential pressure across the filter medium, indicating that the medium is clogged with contaminants. The filtrate compartment is connected to the intake of a filtrate pump to create the pressure differential across the paper medium, and the pump vacuum must be relieved in order to move the filter medium for renewal.
The prior art has suggested several different types of arrangements to relieve the pump vacuum when it is desired to do so. One of the earliest arrangements simply utilized a centrifugal pump, with the pump being shut off so that water in a filtrate storage tank or even in the pressure supply line from the pump could run back through the pump into the filtrate compartment. This arrangement is generally unsatisfactory, since the clean liquid running back into the filtrate compartment at a substantial velocity can lift or tear the paper, releasing dirt into the tank by lifting it from the filter medium. Further, if the vacuum of the filtrate compartment is broken quickly, as by shutting off the pump, the levels of liquid standing in the dirty tank and the elevated clean tank tend to equalize, again pushing liquid upwardly through the filter medium to lift or tear the paper and to force dirt from the paper medium back into the tank. Similar problems are encountered where the filter medium is a loose fibrous filter aid deposited on the foraminous surface of the filtrate compartment. It is necessary to break the vacuum in order to convey the filter aid from the filtrate compartment surface, yet breaking the vacuum too quickly will cause the filter aid to float upwardly into the dirty liquid, while releasing the dirt accreted therein.
Other solutions include interposing a simple swinging check valve between the pump and the filtrate compartment, as in U.S. Pat. No. 4,571,302. The check valve is provided with a small diameter aperture so that some liquid from the clean tank can run back into the filtrate compartment. While this arrangement is satisfactory in some instances, it requires a multiplicity of valves, and the check valve is self-actuating by its own weight, and it is not a positive acting valve.
The present invention proposes a novel valve arrangement which is power-actuated to perform a multiplicity of functions. In a normal filter operating mode, the valve means is positioned in a first position at which the pump is directly connected to the filtrate compartment and flow from the clean liquid reservoir to the pump is prevented. When it is desired to move the filter medium, the valve is power-actuated to a second position at which the valve means accommodates reservoir-pump flow so the pump continues to supply clean liquid to the system, and the valve also permits limited reservoir-filtrate compartment flow to break the filtrate compartment vacuum.
More specifically, the pump communicates with the filtrate compartment by means of a duct which traverses the clean tank, and a single valve having two valving elements is interposed in the duct. A first element controls the flow of clean liquid into the duct, and this element is closed during normal filter operation and is opened during media renewal to supply clean liquid to the pump for circulation to the machining stations. A second valving element is interposed between the filtrate compartment and the duct, and this element is positioned to accommodate full filtrate flow to the pump during normal filtering operation, but is moved to restrict flow of clean liquid from the duct into the filtrate compartment during filter medium renewal, thus breaking the vacuum over a brief time span. The two valving elements are actuated in unison by a single power means, thus providing positive valving action during operation.